Repair: The basic problem is short
circuiting within the transistors used in this model. The old
AF117 and AF114 germanium transistors suffer a crystalline growth
on their substrates which short-circuits the collector to ground.
The resistance of the short varies and is often not serious enough
to stop the particular transistor from completely ceasing to
pass a signal. Battery consumption in this state is very high
and is often the reason for people stopping using the radio.
I opened up the VHF tuner module and found these transistors
to be AF121 and an AF125 which do not suffer from the same shorting
problem. There are 4 IF amplifier transistors in the FM strip
and 3 in the separate AM strip. I cut the connection between
the case/substrate and ground in each of these transistors. This
is a recognised expedient solution to the problem.
I then realigned the FM front end and the associated 10.7MHz
IF amplifier. The VHF receiver now performs very well, albeit
over the restricted tuning range comensurate with the vintage
of the receiver. The AM receiver also performed well after resetting
its oscillator so that the tuning scale read correctly and dealing
with the short in the tuning condenser which I removed by slightly
bending the vanes then adjusting the rear screw to centre the
moving vanes. |
